Nitrogen oxide interaction with copper complexes formed by small peptides belonging to the prion protein octa-repeat region

Abstract

The interaction between NO and copper(II) complexes formed by peptides coming from the N-terminal prion protein octa-repeat region was studied. Aqueous solutions of the Cu-Ac-HGGG-NH₂ and the Cu-Ac-PHGGGWGQ-NH₂ systems around pH 7.5 were tested after the addition of NONOates as a source of NO. UV-Vis, room temperature and frozen solution EPR spectra showed the occurrence of copper(II) reduction in all these complexes. The reduction of these complexes is probably mediated by the formation of a labile NO adduct, which, after re-oxidation, leads to a relatively stable NO₂⁻ adduct through the apical coordination along the void site of their square pyramidal structure. In fact, the most significant shifts in EPR magnetic parameters (g_‖ and A_‖ or g_iso and A_iso) as well as in the optical parameters (λ_max and ε_max) gave a reason for geometrical changes of the copper coordination polyhedron from a distorted square pyramid to a pseudo-octahedron. The presence of oxygen in the aqueous solution hindered the reduction ability of NO towards copper, but it made it easier to return to the original species. In order to elucidate the possible mechanism of this interaction, the reduction of copper complexed by these ligands was followed by means of zinc powder addition. The further addition of nitrite to the solution containing reduced copper led to the conclusion that nitrite could easily form an adduct, which after re-oxidation presented the same spectral features of the species obtained when the NO interaction was followed. The complexity of this interaction could involve both an inner or an outer-sphere electron transfer mechanism.